1. Field of the Invention
The present invention relates to an ultra-high pressure discharge lamp having remarkably improved pressure resistance.
2. Description of the Related Art
Recently, ultra-high pressure discharge lamps have been frequently used not only as light sources in liquid crystal projectors for projecting images but also as optical fiber light sources for use in optical devices including medical instruments. Particularly, such ultra-high pressure discharge lamps for use as light sources in liquid crystal projectors are required to be smaller point sources and to exhibit a higher luminance than presently existing light sources in order to ensure clearer and brighter images for a prolonged time. Lamp makers are in fierce competition for satisfying these demands. As a result, at present, the internal volume of an arc tube of an envelope is decreasing gradually and the required internal pressure within such an arc tube upon lighting is approximating to the limit of the strength of the material forming the envelope.
A recent ultra-high pressure discharge lamp calling for such a high pressure is susceptible to lamp explosion upon lighting due to a trivial defect caused during its manufacture. In view of this problem, Japanese Patent Laid-Open Gazette No. HEI 11-118753 has provided an ultra-high pressure discharge lamp wherein a metal foil of each mount to be embedded in a seal portion of an envelope and welded portions of electrode and external lead pin welded to the metal foil are presealed with a preseal glass member; the presealed portion is embedded in the seal portion; and the preseal glass member is wholly fused to the inner peripheral surface of the seal portion to make the preseal glass member and the seal portion integral with each other. In some cases such a lamp can attain a pressure resistance of 140 atm or higher.
However, problems in respect of the lamp reliability and the fraction defective still exist for the following reason. That is, since the preseal glass member embedding the metal foil portion of each mount (formed by welding the metal foil to an electrode at one end thereof and to a lead pin at the opposite end thereof) and the glass material of the seal portion of the envelope are both made of quartz, fusing can be achieved more easily than in the case where the metal foil is directly embedded in the seal portion.
In this case the amount of thermal expansion of metal portions such as the electrode and the external lead pin increases with increasing heating time. Particularly, the process employed to seal the whole seal portion as in the prior art is likely to cause a number of fine cracks to occur at the welded portion between the electrode and the metal foil (see cracks (C) in the enlarged view at FIG. 1). Such fine cracks may initiate lamp explosion and hence may-cause the pressure resistance of the lamp to be lowered substantially. If the lamp explodes, not only broken pieces of glass but also the halogen gas and mercury encapsulated within the lamp are scattered around, thus resulting in a problem of contamination of the surrounding environment.
Accordingly, it is an object of the present invention to provide an ultra-high pressure discharge lamp having seal portions of a structure capable of realizing a pressure resistance of 180 atm or higher while ensuring a higher yield.
It is another object of the present invention to provide an ultra-high pressure discharge lamp having seal portions of a structure capable of preventing scattering of encapsulated substances upon lamp explosion.
According to the present invention, there is provided an ultra-high pressure discharge lamp comprising: a pair of mounts each comprising a metal foil having one end welded to an associated electrode and an opposite end welded to an associated external lead pin, and a preseal glass member embedding the metal foil therein; and a quartz envelope having an arc tube portion provided with a pair of seal portions on opposite sides thereof, the electrodes associated with the respective mounts being disposed opposite to each other in the arc tube portion, the preseal glass member being hermetically sealed in a respective one of the seal portions, wherein
the preseal glass member has a portion fused to an inner peripheral surface of the respective seal portion over an entire circumference thereof.
With this construction in which the preseal glass member has a portion fused to an inner peripheral surface of the respective seal portion over an entire circumference thereof, fusing can be completed in a shorter time than in the case where the whole preseal glass member is fused to the inner peripheral surface of the seal portion, so that the resulting thermal strain can be reduced. The reduction in thermal strain leads to a reduction in the development of fine cracks at the fused portion where the preseal glass member and the inner peripheral surface of the seal portion are fused together, particularly at a portion of the preseal glass member in contact with the root portion of an electrode shank.
Particularly, the possibility of lamp explosion initiated from the root portion can be reduced substantially. As a result, the yield of ultra-high pressure discharge lamp can be improved substantially. In addition, the ultra-high pressure discharge lamp thus constructed requires a substantially shortened heating time as compared with prior-art ultra-high pressure discharge lamps. The shortened heating time can contributes to energy saving and improvement in the dimensional precision in respect of the electrodes.
Preferably, the fused portion of the preseal glass member is an end portion of the preseal glass member situated adjacent the associated electrode.
This feature allows the will-be fused portion of the preseal glass to be fused to the inner peripheral surface of the seal portion in a shorter time and, as a result, it is possible to inhibit the occurrence of fine cracks, which are likely to result from possible overheating as described above, as well as to minimize thermal strain.
Preferably, the preseal glass member has two portions fused to the inner peripheral surface of the seal portion, one being an end portion situated adjacent the associated electrode, the other being an end portion situated adjacent the associated external lead pin.
With this feature, even if fine cracks occur at the preseal glass member, particularly at the portion of the preseal glass member in contact with the root portion of the electrode shank, and grow to reach the clearance between the preseal glass member and the respective seal portion thereby permitting the gas and other substances encapsulated within the arc tube to leak, the fused portion at the end portion of the preseal glass member situated adjacent the associated external lead pin blocks the leaking substances thereby preventing the encapsulated substances such halogen gas and mercury from leaking out of the lamp. Thus, contamination of the surroundings which is possible upon lamp explosion can be avoided.
Preferably, the fused portion of the preseal glass member at the end portion situated adjacent the associated electrode has a length equal to or larger than a wall thickness of a respective one of the seal portions, provided that the length is not larger than 10 mm.
By setting the length of the fused portion to a value equal to or larger than the wall thickness of the respective seal portion the pressure resistance of the fused portion can be made substantially equal to or higher than that of the seal portion. However, if the wall thickness of the seal portion becomes larger than 10 mm, the seal portion is likely to be overheated, which may cause fine cracks to occur as described above.
Preferably, the preseal glass member comprises a preseal tube having a wall thickness smaller than a wall thickness of a respective one of the seal portions.
This feature makes it possible to shorten the heating time thereby inhibiting the occurrence of fine cracks and reducing the thermal strain as in the above-described cases.
The foregoing and other objects, features and attendant advantages of the present invention will become apparent from the reading of the following detailed description in connection with the accompanying drawings.